Magnetic resonance imaging of arterial structures

ABSTRACT

A medical imaging system (100, 1200) includes a memory (136) for storing machine executable instructions (170), and a processor (130) for controlling the medical imaging system. Execution of the machine executable instructions causes the processor to: receive (304, 1000) a tagged arterial spin labeled (ASL) magnitude image (148) of a region of interest of a subject (118); receive (306, 1002) a control ASL magnitude image (150) of the region of interest of the subject; construct (308, 1004) an arterial image (152) by subtracting the control ASL magnitude image and the tagged ASL magnitude image; construct (310, 1006) an arterial mask (154) using the arterial image by identifying arteries in the arterial image; receive (312, 1008) a phase contrast magnetic resonance image (156, 158, 160) of at least a portion of the region of interest of the subject; and construct a venous image (164) at least partially by setting voxels of the phase contrast image within the at least a portion of the region of interest that are within the arterial mask to a predetermined background value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application of International Application No. PCT/EP2017/058482, filed on Apr. 10, 2017, which claims the benefit of EP Application Serial No. 16166446.1 filed on Apr. 21, 2016 and is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to magnetic resonance imaging, in particular the invention is related to arterial spin tagging magnetic resonance imaging techniques.

BACKGROUND OF THE INVENTION

A large static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the B0 field.

During an MRI scan, Radio Frequency (RF) pulses generated by one or more transmitter coils cause a called B1 field. Additionally applied gradient fields and the B1 field cause perturbations to the effective local magnetic field. RF signals are then emitted by the nuclear spins and detected by one or more receiver coils. These RF signals are used to construct the MR images. These coils can also be referred to as antennas.

MRI scanners are able to construct images of either slices or volumes. A slice is a thin volume that is only one voxel thick. A voxel is a small volume element over which the MR signal is averaged, and represents the resolution of the MR image. A voxel may also be referred to as a pixel (picture element) herein if a single slice is considered.

By performing different magnetic resonance imaging protocols (which are implemented as pulse sequences or pulse sequence commands), different types of information can be measured about a subject. For example, there are various techniques, which enable the encoding of spins such that the flow or diffusion of fluid can be directly measured. Arterial spin tagging is a technique where the spins of blood passing through a group of arteries or even single arteries can be labeled and then imaged. The reference book “Handbook of MRI Pulse Sequences” (hereafter “Handbook of MRI Pulse Sequences”) by Bersnstein et. al., Elsevier, 2004, ISBN 978-0-12-092861-3 describes in section 17.1 (pp. 802 through 829) provides a review of several different arterial spin tagging techniques.

The journal article “In Vivo Estimation of the Flow-Driven Adiabatic Inversion Efficiency for Continuous Arterial Spin Labeling: A Method Using Phase Contrast Magnetic Resonance Angiography,” O'Gorman et. al., Magnetic Resonance in Medicine 55:1291-1297 (2006) describes the combination of arterial spin labeling with an estimation of the flow-driven adiabatic inversion efficiency. Axial velocity maps are acquired using a peripherally gated 2D triggered phase contrast sequence and are acquired separately from perfusion measurements using a multi-slice CASL technique.

The journal article Wu, Wen-Chau, and Eric C. Wong. “Intravascular effect in velocity-selective arterial spin labeling: the choice of inflow time and cutoff velocity.” Neuroimage 32.1 (2006): 122-12, DOI: 10.1016/j.neuroimage.2006.03.001, discloses a velocity-selective arterial spin labeling (VS-ASL) tags spins on a basis of flow velocity, instead of spatial distribution that has been commonly adopted in conventional ASL techniques. VS-ASL can potentially generate tags that are very close to the imaging plane and whereby avoid the error source of transit delay (δt) variation independent of inflow time (TI). In practice, however, TI of VS-ASL should still be chosen with caution with respect to intravascular signal and cutoff velocity (Vc). The presented study takes advantage of multiple TI and Vc to systematically investigate the intravascular effect.

The book section Haacke, E. Mark, et al. “11.2 Continous Properties and Phase Imaging” In: Magnetic resonance imaging: physical principles and sequence design. Vol. 82. New York:: Wiley-Liss, 1999, ISBN: 978-0471351283 provides a review of phase imaging in magnetic resonance imaging.

The journal article Jensen-Kondering, Ulf, et al. “Superselective pseudo-continuous arterial spin labeling angiography.” European journal of radiology 84.9 (2015): 1758-1767, DOI: 10.1016/j.ejrad.2015.05.034, discloses the evaluation of the utility of a novel non-contrast enhanced, vessel-selective magnetic resonance angiography (MRA) approach based on superselective pseudo-continuous arterial spin labeling (ASL) for the morphologic assessment of intracranial arteries when compared to a clinically used time-of-flight (TOF) MRA.

SUMMARY OF THE INVENTION

Embodiments of the invention provide for an improved analysis of arterial spin tagging images that combines information from phase contrast images with Arterial Spin Labeled (ASL) magnitude images. ASL is a common abbreviation that is synonymous with arterial spin tagging. Magnitude images can be reconstructed to locate arteries by detecting blood flow that has been tagged. The phase images may be used to detect the flow of fluid such as blood either inside or outside of the arteries located by the ASL magnitude images.

Combining the phase information with the data from the magnitude images can provides detailed information about both the venous and arterial system. For example the magnitude images can be used to infer the location of tagged arteries directly. A knowledge of the location of the arteries from these tagged images can be used to create a mask which defines the location of known arteries. Flow information in the phase images which is within the location defined by the arteries can be attributed to flow within these arteries. Phase maps can then be used to determine the flow patterns of blood through arteries identified by the arterial mask.

Flow information within the phase images outside of the arterial masks can be attributed to flow within veins (or possibly untagged arteries). The phase maps can then be used to make a map of venous blood flow. It is even possible to make composite images which detail both arterial and venous blood flow.

Magnetic Resonance (MR) data is defined herein as being the recorded measurements of radio frequency signals emitted by atomic spins using the antenna of a Magnetic resonance apparatus during a magnetic resonance imaging scan. Magnetic resonance data is an example of medical image data. A Magnetic Resonance Imaging (MRI) image is defined herein as being the reconstructed two or three dimensional visualization of anatomic data contained within the magnetic resonance imaging data. This visualization can be performed using a computer.

In one aspect the invention provides for a medical imaging system. The medical imaging system comprises a memory for storing machine-executable instructions. The medical imaging system further comprises a processor for controlling the medical imaging system. Execution of the machine-executable instructions causes the processor to receive a tagged ASL (arterial spin labeling) magnitude image of a region of interest of a subject. The tagged ASL magnitude image may also be referred to as the tagged magnitude image herein. The tagged ASL magnitude image may also be referred to as a tagged ASL magnitude magnetic resonance image. Execution of the machine-executable instructions further cause the processor to receive a control ASL magnitude image of the region of interest of the subject. The control ASL magnitude image may also be referred to as the control magnitude image herein. The control ASL magnitude image may also be referred to as a control ASL magnitude magnetic resonance image. Execution of the machine-executable instructions further cause the processor to construct an arterial image by subtracting the tagged ASL magnitude image and the control ASL magnitude image. The tagged ASL magnitude image and the control ASL magnitude image may for instance have been acquired according to an arterial spin labeling or arterial spin tagging magnetic resonance imaging protocol.

The tagged ASL magnitude image and the control ASL magnitude image may have a one-to-one correspondence in the number and size of voxels that comprise the images. Execution of the machine-executable instructions further cause the processor to construct an arterial mask using the arterial image by identifying arteries in the arterial image. Execution of the machine-executable instructions further cause the processor to receive a phase-contrast magnetic resonance image with at least a portion of the region of interest of the subject. In some examples the phase-contrast magnetic resonance image may have the same region of interest as the tagged ASL magnitude image and the control ASL magnitude image. In other cases the region of interest of the phase-contrast magnetic resonance image may only partially overlap the region of interest of the tagged ASL magnitude image and the control ASL magnitude image. A phase contrast magnetic resonance image as used herein may also be referred to as a “phase image.”

Execution of the machine-executable instructions further cause the processor to construct a selected phase-contrast arterial image at least partially by setting voxels of the phase-contrast image within at least a portion of the region of interest that are outside of the arterial mask to a predetermined background value. In some further examples the region of the phase-contrast magnetic resonance image which is outside of the region of interest of the tagged ASL magnitude image or the control ASL magnitude image is also set to the predetermined background value.

This embodiment may have the benefit of providing a means for constructing a phase-contrast arterial image that may be useful for showing for example the blood flow through arteries.

In some examples, the arterial mask may be applied to multiple phase-contrast magnetic resonance images. In this case the multiple phase-contrast magnetic resonance images may be summed after the arterial mask has been applied. In some cases the multiple phase-contrast magnetic resonance images may be summed and then the arterial mask may be applied.

In another embodiment, execution of the machine-executable instructions further cause the processor to construct a venous image at least partially by setting the voxels of the phase-contrast image within at least a portion of the region of interest that are within the arterial mask to the predetermined background value. This embodiment may be beneficial because the prior knowledge of the arterial mask may be used to generate a venous image using the same data which was used for generating the arterial image.

In another embodiment execution of the machine-executable instructions further cause the processor to calculate a composite image by combining the arterial image and the venous image into a composite image. In some instances the composite image may use different colors and/or color maps to indicate which is the venous and which is the arterial portion of the image. Variations in the color map may also be used to indicate the velocity of flow within the composite image.

In another embodiment, the medical imaging system is a magnetic resonance imaging system.

In another embodiment, the memory further contains phase-contrast pulse sequence commands.

In another embodiment, the memory further contains tagging pulse sequence commands. The memory further contains control pulse sequence commands. The tagging pulse sequence commands comprise a tagging inversion pulse portion for spin labeling a tagging location within the subject. The control pulse sequence commands comprise a control inversion pulse portion. The phase-contrast pulse sequence commands comprise a phase-contrast readout portion. The control pulse sequence may include a motion-encoding gradient in a readout direction. The phase-contrast readout portion comprises phase-contrast encoding in at least one direction. The at least one direction may include the readout direction. The phase-contrast readout portion may of course have phase-contrast encoding in multiple directions.

The tagging pulse sequence commands and the control pulse sequence commands are used for arterial spin tagging as is described in section 17.1 of the Handbook of MRI Pulse Sequences. The tagging inversion pulse portion is used to label a bolus of blood that is travelling through one or more arteries. The tagging inversion pulse portion results in a magnetization transfer effect that will be visible in a magnitude image calculated from magnetic resonance data that is acquired using the tagging pulse sequence commands. The control inversion pulse portion is constructed so that it causes a magnetization transfer effect that is equivalent or nearly equivalent to the magnetization transfer effect caused by the tagging inversion pulse portion.

Execution of the machine-executable instructions further cause the processor to acquire tagged magnetic resonance data by controlling the magnetic resonance imaging system with the tagging pulse sequence commands. Execution of the machine-executable instructions further cause the processor to acquire control magnetic resonance data by controlling the magnetic resonance imaging system with the control pulse sequence commands. Execution of the machine-executable instructions further cause the processor to reconstruct the tagged ASL magnitude image using the tagged magnetic resonance data. The tagged ASL magnitude image is received by reconstructing the tagged ASL magnitude image. Execution of the machine-executable instructions further cause the processor to reconstruct the control ASL magnitude image using the control magnetic resonance data. The control ASL magnitude image is received by reconstructing the control ASL magnitude image.

Execution of the machine-executable instructions further cause the processor to acquire phase-contrast magnetic resonance data by controlling the magnetic resonance imaging system with the phase-contrast pulse sequence commands. Execution of the machine-executable instructions further cause the processor to reconstruct the phase-contrast image using the phase-contrast magnetic resonance data at least partially using the phase encoding in the at least one direction. The phase-contrast image is received by reconstructing the phase-contrast image.

In another embodiment, the medical imaging system is a magnetic resonance imaging system. The memory further contains tagging pulse sequence commands and control pulse sequence commands. The tagging pulse sequence commands comprise a tagging inversion pulse portion for spin labeling a tagging location within the subject. The tagging pulse sequence commands comprise a phase-contrast readout portion. The phase-contrast readout portion comprises phase-contrast encoding in the at least one direction. The at least one direction may include the readout direction. The control pulse sequence commands comprise a control inversion pulse portion. The control pulse sequence commands comprise the phase-contrast readout portion.

Execution of the machine-executable instructions further cause the processor to acquire tagged magnetic resonance data by controlling the magnetic resonance imaging system with the tagging pulse sequence commands. Execution of the machine-executable instructions further cause the processor to acquire a control ASL magnetic resonance data by controlling the magnetic resonance imaging system with the control pulse sequence commands. Execution of the machine-executable instructions further cause the processor to reconstruct the tagged ASL magnitude image using the tagged magnetic resonance data. The tagged ASL magnitude image is received by reconstructing the tagged ASL magnitude image. Execution of the machine-executable instructions further cause the processor to reconstruct the control magnitude image using the control magnetic resonance data. The control ASL magnitude image is received by reconstructing the control ASL magnitude image. Execution of the machine-executable instructions further cause the processor to reconstruct the phase image using either the tagged magnetic resonance data and/or the control magnetic resonance data at least partially using phase encoding in the at least one direction. The phase-contrast image is received by reconstructing the phase-contrast image. The phase encoding in the at least one direction may possibly be interpreted as phase-contrast encoding in at least one direction. In some examples the at least-one direction may include a readout direction.

In another embodiment the tagging location is selected for a single artery.

In another embodiment the tagging location is configured for being selective for a single artery.

In another embodiment execution of the machine-executable instructions cause the processor to repeat acquiring the tagged magnetic resonance data and the control magnetic resonance data for a predetermined number of tagging volumes. Execution of the machine-executable instructions further causes the processor to calculate the arterial image for each of the predetermined number of tagging volumes.

In another embodiment the tagging location is non-selective.

In another embodiment the tagging location is configured for being non-selective.

In another embodiment execution of the tagging inversion pulse portion by the magnetic resonance imaging system causes a tagging magnetization transfer effect in the tagged magnitude image. Execution of the control inversion pulse portion by the magnetic resonance imaging system causes a control magnetization transfer effect in the control magnitude image. The tagging magnetization transfer effect is subtracted out of the arterial image by the control magnetization transfer effect.

In another embodiment execution of the tagging inversion pulse portion by the magnetic resonance imaging system is configured for causing a tagging magnetization transfer effect in the tagged magnitude image. Execution of the control inversion pulse portion by the magnetic resonance imaging system is further configured for causing a control magnetization transfer effect in the control magnitude image such that the tagging magnetization transfer effect is able to be subtracted out of the arterial image by the control magnetization transfer effect.

Execution of the machine-executable instructions further cause the processor to receive a registration between the phase-contrast image and the control ASL magnitude image and/or tagged ASL magnitude image. The identification of the voxels in the phase-contrast image within the at least a portion of the region of interest that are outside the arterial map to a predetermined background value is at least partially performed using the registration. This embodiment may be useful when the volume or region of interest imaged by the phase-contrast magnetic resonance image is not identical with the region of interest of the tagged ASL magnitude image and/or the control ASL magnitude image. Receiving registration between the two images may allow the phase-contrast image to be used even though the regions of interest are not identical.

In another embodiment execution of the machine-executable instructions further cause the processor to receive the registration between the phase-contrast image and the control ASL magnitude image and/or the tagged ASL magnitude image by calculating a mapping between voxels of the phase-contrast image and the control ASL magnitude image and/or the tagged ASL magnitude image. If there is a mapping between the phase-contrast image and either of the control ASL magnitude images then the mask may be applied. For example if the mapping is known then this can be used for translating or using the arterial mask to properly set voxels to the predetermined background level.

In another embodiment, the arterial mask is created using the arterial image according to any one of the following: by thresholding the arterial image using a predetermined threshold value. In this case portions of the arterial image above the predetermined threshold value are identified as being arterial regions. The arterial mask may also be created by segmenting the arterial image. The arterial mask may also be constructed by applying an anatomical atlas to the arterial image. The arterial mask may also be created by fitting a deformable shape model to the arterial image. The arterial image may also be created according to any one of the accommodation of the aforementioned techniques.

In another aspect, the invention provides for a computer program product comprising machine-executable instructions for execution by a processor. Execution of the machine-executable instructions further cause the processor to receive a tagged ASL magnitude image of a region of interest of a subject. Execution of the machine-executable instructions further cause the processor to receive a control ASL magnitude image of the region of interest of the subject. Execution of the machine-executable instructions further cause the processor to construct an arterial image by subtracting the tagged ASL magnitude image from the control ASL magnitude image.

Execution of the machine-executable instructions further cause the processor to construct an arterial mask using the arterial image by identifying arteries in the arterial image. Execution of the machine-executable instructions further cause the processor to receive a phase-contrast magnetic resonance image of at least a portion of the region of interest of the subject. Execution of the machine-executable instructions further cause the processor to construct a selected phase-contrast arterial image at least partially by setting voxels of the phase-contrast image within at least a portion of the region of interest but outside of the arterial mask to a predetermined background value.

In another aspect, the invention provides for a computer program product comprising machine-executable instructions for execution by a processor. Execution of the machine-executable instructions further cause the processor to receive a tagged ASL magnitude image of a region of interest of a subject. Execution of the machine-executable instructions further cause the processor to receive a control ASL magnitude image of the region of interest of the subject. Execution of the machine-executable instructions further cause the processor to construct an arterial image by subtracting the tagged ASL magnitude image from the control ASL magnitude image.

Execution of the machine-executable instructions further cause the processor to construct an arterial mask using the arterial image by identifying arteries in the arterial image. Execution of the machine-executable instructions further causes the processor to receive a phase-contrast magnetic resonance image of at least a portion of the region of interest of the subject. Execution of the machine-executable instructions further cause the processor to construct a venous image at least partially by setting voxels of the phase-contrast image within the at least a portion of the region of interest that are within the arterial mask to a predetermined background value.

In another aspect, the invention provides for a method of medical imaging. The method comprises receiving a tagged ASL magnitude image of a region of interest of a subject. The method further comprises receiving a control ASL magnitude image of the region of interest of the subject. The method further comprises reconstructing an arterial image by subtracting the tagged ASL magnitude image from the control ASL magnitude image. The method further comprises constructing an arterial mask using the arterial image by identifying arteries in the arterial image. The method further comprises receiving a phase-contrast magnetic resonance image of at least a portion of the region of interest of the subject. In some examples the region of interest of the phase-contrast magnetic resonance image is identical with the region of interest of the tagged ASL magnitude image and/or the control ASL magnitude image. However it does not need to be. The method further comprises constructing a selected phase-contrast arterial image at least partially by setting voxels of the phase-contrast image within the at least a portion of the region of interest that are outside of the arterial mask to a predetermined background value.

In an example, the invention provides for a magnetic resonance imaging system for imaging a subject. The subject may for instance be at least partially within an imaging zone of the magnetic resonance imaging system. The magnetic resonance imaging system comprises a memory for storing machine-executable instructions. The memory further contains tagging pulse sequence commands and control pulse sequence commands. The tagging pulse sequence commands comprise a tagging inversion pulse portion for spin labeling a tagging location within the subject. The tagging pulse sequence commands further comprise a phase-contrast readout portion. The phase-contrast readout portion comprises a phase-contrast encoding in at least one direction. The control pulse sequence commands comprise a control inversion pulse portion. The control pulse sequence commands comprise the phase-contrast readout portion.

The tagging pulse sequence commands and the control pulse sequence commands are used for arterial spin tagging as is described in section 17.1 of the Handbook of MRI Pulse Sequences. The tagging inversion pulse portion is used to label a bolus of blood that is travelling through one or more arteries. The tagging inversion pulse portion results in a magnetization transfer effect that will be visible in a magnitude image calculated from magnetic resonance data that is acquired using the tagging pulse sequence commands. The control inversion pulse portion is constructed so that it causes a magnetization transfer effect that is equivalent or nearly equivalent to the magnetization transfer effect caused by the tagging inversion pulse portion.

The magnetic resonance imaging system further comprises a processor for controlling the magnetic resonance imaging system. Execution of the machine-executable instructions causes the processor to acquire tagged magnetic resonance data by controlling the magnetic resonance imaging system with the tagging pulse sequence commands. The tagged magnetic resonance data may for instance be acquired from a region of interest. Execution of the machine-executable instructions further cause the processor to acquire control magnetic resonance data by controlling the magnetic resonance imaging system with the control pulse sequence commands. The control magnetic resonance data may be acquired from the same region of interest as the tagged magnetic resonance data.

Execution of the machine-executable instructions further cause the processor to reconstruct a tagged magnitude image using the tagged magnetic resonance data. Execution of the machine-executable instructions further cause the processor to reconstruct a control magnitude image using the control magnetic resonance data.

Execution of the machine-executable instructions further cause the processor to reconstruct a using either the tagged magnetic resonance data and/or the control magnetic resonance data. The phase image is at least partially reconstructed using the phase encoding at least one direction. There may be a phase image reconstructed for each of the at least one direction.

In this example the location of the arteries may be determined using the magnitude images and the location of veins are located using the phase images. The phase encoding is not affected by the tagging inversion pulse portion and the control inversion pulse portion. Therefore either the tagged magnetic resonance data and/or the control magnetic resonance data may be used to create the first, second and third phase images. In some instances it may be beneficial to use both the tagged magnetic resonance data and the control magnetic resonance data to increase the signal-to-noise ratio in the first phase image, the second phase image, and the third phase image.

In another example, the at least one direction is a first direction, a second direction, and a third direction. The first direction, the second direction, and the third direction may be orthogonal. However, it is not necessary for the first direction, the second direction, and the third direction to be orthogonal. This example may be beneficial because magnetic resonance imaging systems are normally fitted with three orthogonal magnetic field gradients. It may be efficient and produce good imaging results when the three directions are chosen to correspond to the orientation of the magnetic field gradient coils.

In another example, execution of the machine-executable instructions further cause the processor to reconstruct a first phase image, a second phase image and a third phase image using either the tagged magnetic resonance data and/or the control magnetic resonance data. The first phase image is reconstructed using the phase encoding in the first direction. The second phase is reconstructed using the phase encoding in the second direction. The third phase image is reconstructed using the phase encoding in the third direction. The best results will be obtained when the first direction, the second direction and the third direction are orthogonal. However, the example described herein will still function when the first direction, the second direction, and the third direction are not orthogonal.

It may also be beneficial to use more than three directions. The phase contrast information from multiple images can be combined to improve the image quality.

In another example execution of the machine executable instructions further causes the processor to construct a selective phase contrast arterial image at least partially by setting voxels of the phase image that are outside of the arterial mask to a predetermined background value. If there are multiple directions this may be performed for phase images reconstructed in each of the directions to create intermediate arterial phase contrast images.

In another example Execution of the machine-executable instructions further cause the processor to construct an arterial image by subtracting the tagged magnitude image from the control magnitude image. Execution of the machine-executable instructions further cause the processor to construct an arterial mask using the arterial image by identifying arteries in the arterial image. The identification of the arteries may be performed in a number of different ways. The arterial image contains voxels with a larger value than the majority of the other voxels. These will most likely be portions of an artery. The location of arteries could therefore in one example be performed by thresholding the image. In other examples it may be possible to examine the connectedness of arteries. For example, it is expected that arteries would follow a path through which blood would flow. Voxels which are brighter than normal or above the threshold but are not connected to other voxels may in some instance be disregarded. In other examples anatomical models such as from an atlas or shape deformable models may be fit to the arterial image.

A mask as used herein encompasses a defined or identified region of an image or visualization of medical image data. For example, the arterial mask is constructed using the arterial image. The arterial mask is used to identify voxels or pixel within images that are identified as being part or partially composed of identified arteries. The arterial mask can then be applied or compared to other images and used to determine if the voxels in the other image contain identified arteries or not. If a voxel in an image is “within” or “inside” the arterial mask, then it contains an indentified artery or part of an identified artery. If a voxel is “outside” of the arterial mask then the voxel does not contain or partially contain an identified artery. Implicit in this is that there is known relationship between voxels of the image used to create the mask and the image to which the mask is being applied to. Normally, this is a one to one relationship between the voxels, however this is not necessary. When working with images produced by a single medical imaging system such as a single magnetic resonance imaging system the relationship or mapping between voxels in different images is well known.

In another example execution of the machine executable instructions further causes the processor to construct a first intermediate arterial phase contrast image by setting voxels of the first phase image that are outside of the arterial mask to a predetermined background value. Execution of the machine executable instructions further cause the processor to construct a second intermediate arterial phase contrast image by setting voxels of the second phase image that are outside of the arterial mask to a predetermined background value. Execution of the machine executable instructions further cause the processor to construct a third intermediate arterial phase contrast image by setting voxels of the third phase image that are outside of the arterial mask to a predetermined background value. Execution of the machine executable instructions further cause the processor to calculate a selective phase contrast arterial image by adding the first intermediate arterial phase contrast image, the second intermediate arterial phase contrast image, the third intermediate arterial phase contrast image. This example may have the advantage that a selective phase contrast arterial image is constructed where the flow and tagging information are acquired simultaneously. This may for example reduce the acquisition time. It may also reduce the amount of motion artifacts.

In another example, execution of the machine-executable instructions further cause the processor to calculate a venous image at least partially by setting voxels within the venous image that correspond to or are within the arterial mask to the predetermined background value.

In another example, execution of the machine-executable instructions further cause the processor to calculate a combined phase image by adding the first phase image, the second phase image, and the third phase image. Execution of the machine-executable instructions further cause the processor to calculate a venous image by setting all voxels within the combined phase image that are above a predetermined value and are also outside of the arterial mask. This may provide for a robust means of calculating the location of veins.

In another example, the venous image is a selective phase contrast venous image. Execution of the machine executable instructions further cause the processor to construct a first intermediate venous phase contrast image by setting voxels of the first phase image that are outside of the arterial mask to a predetermined background value. Execution of the machine executable instructions further cause the processor to construct a second intermediate venous phase contrast image by setting voxels of the second phase image that are outside of the arterial mask to a predetermined background value. Execution of the machine executable instructions further cause the processor to construct a third intermediate venous phase contrast image by setting voxels of the third phase image that are outside of the arterial mask to a predetermined background value. Execution of the machine executable instructions further cause the processor to calculate the venous image by adding the first intermediate venous phase contrast image, the second intermediate venous phase contrast image, the third intermediate venous phase contrast image.

In the above examples, the tagging pulse sequence commands and the control pulse sequence commands are used to perform arterial spin tagging. The addition of the phase-contrast readout portion to both the tagging inversion pulse portion and the control inversion pulse portion enables the location of the veins to be determined using a single arterial spin tagging acquisition. The arteries are located in the magnitude images. This provides a knowledge of where the arteries are. This enables those particular voxels which belong to the arteries to then be excluded from the combined phase images. This may provide for an improved means of detecting the location of both arteries and veins within a subject.

In another example the venous image and the arterial image are combined into a single image. In some examples false colors or other color scales may be used to indicate which are the venous and arterial portions of the image.

In another example the venous image and the selective phase contrast arterial image are combined into a single image. In some examples false colors or other color scales may be used to indicate which are the venous and arterial portions of the image, and also the magnitude of flow through the arteries or veins.

In another example the arterial image and the venous image are both three-dimensional images. In other examples they may be two-dimensional slabs or stacks of two-dimensional images. In other words the above example may be performed as a two-dimensional, a series of two-dimensional acquisitions, or as a three-dimensional magnetic resonance imaging protocol.

In another example the tagging volume is selectively positioned to encompass a cross-section of a single artery. The tagging volume may then be interpreted as selective labeling. This example may be beneficial because the blood flow to the region of interest may be mapped for a single artery.

In another example execution of the machine-executable instructions further cause the processor to repeat acquiring the tagged magnetic resonance data and the control magnetic resonance data for different tagging volumes. This for instance may be performed for a predetermined number of tagging volumes. Execution of the machine-executable instructions further causes the processor to calculate the arterial image for each of the predetermined number of tagging volumes. This example may be useful in imaging a region which is supplied by more than one artery. For instance the brain is supplied by a number of different arteries. The blood supply for each of the individual arteries may be imaged separately.

When more than one artery is imaged, the arterial mask may be constructed using a composite of all the reconstructed arterial images for each of the different arteries. Likewise, the venous image may be reconstructed from the tagged magnetic resonance data and/or the control magnetic resonance data for one, all, or a combination of the magnetic resonance data acquired for the different arteries.

In another example the tagging location is a tagging volume defined by one or more tagging voxels. In some examples this may be voxels identified in a preliminary magnetic resonance image which is used to determine the location of different arteries. Various voxels can be selected such that different arteries are selected.

In another example the tagging location is non-selective. For example the tagging location may be a tagging plane. For example, when imaging the blood flow into the brain a plane may be selected which tags all of the arteries feeding the brain simultaneously. In this case the arterial image images the total blood supply to the brain and the venous image images the veins which are fed by these arteries.

In another example execution of the machine-executable instructions further causes the processor to receive the tagging location. For example the processor may receive the tagging location via a user interface. In other cases the tagging location may be received automatically. For instance a preliminary medical image may provide or contain data which is descriptive of the location of arteries within a subject. An operator could manually delineate where the one or more tagging locations are within the subject. In other instances a system may perform this automatically.

In another example the tagging location is received by segmenting a preliminary medical image. The preliminary medical image may be acquired using a different imaging modality such as a PET scan or a CT image. In other examples the preliminary medical image may be a scout scan or other image which is performed first with the subject already in location in the magnetic resonance imaging system. The segmentation algorithm may automatically identify the location of arteries or it may be simply used to position a tagging plane for non-selective tagging.

In another example the venous image and the arterial image are both any one of the following: a planar image, a stack of planar images, and a three-dimensional image. That is to say the example may be applied to a two-dimensional acquisition, a number of two-dimensional acquisitions or a full three-dimensional magnetic resonance imaging protocol.

In another example execution of the machine-executable instructions cause the processor to calculate a composite image by combining the arterial image and the venous image into a composite image. The composite image may take different forms. In some examples different colors may be used to indicate the velocity of blood flow and/or whether it is arterial and/or venous. It may be beneficial to combine the arterial image and the venous image into one image as it may provide a better picture of the blood supply and drain from a region of interest. In particular, in imaging brain scans, it may help a practitioner to identify abnormal conditions in the subject more readily.

In another example the multiple arterial images are acquired for different tagging volumes. These various arterial images may be combined also into a composite image which may be combined possibly with the venous image also. In a case where more than one tagging volume is used and they are selectively used to label different the arteries, the blood supply for different arteries may be labeled so it is easier to visualize the blood supply supplied by different arteries in relation to each other and also the vein system.

In another example the arteries and the veins in the composite image are differentiated by color or different color scales.

In another example the tagging pulse sequence commands comprise a pre-saturation portion before the tagging inversion pulse portion. The control pulse sequence commands comprise the pre-saturation portion before the control inversion pulse portion. The pre-saturation portion may be used to kill the magnetization of spins within a region of interest. Using the pre-saturation portion may provide better results when constructing the arterial image. It may provide for better signal-to-noise and also remove image artefacts. However, it is not necessary to perform the pre-saturation portion in the tagging pulse sequence commands and the control pulse sequence commands.

In another example the arterial mask is created by thresholding the arterial image using a predetermined threshold value. Portions of the arterial image above the predetermined threshold value are identified as being arterial regions.

In another example the arterial mask is at least partially created by segmenting the arterial image. A segmentation algorithm may be used to identify arteries within the arterial image. For instance an origin of the artery may be known and the segmentation algorithm may use the value of the pixel voxels in comparison with them being connected to each other.

In another example the arterial mask is created at least partially by applying an anatomical atlas to the arterial image. For example the arteries within subjects with normal anatomy may be similar that anatomical atlases may be used to help identify which portions of the arterial image are arterial.

In another example the arterial mask is at least partially created by fitting a deformable shape model to the arterial image. For example the deformable shape model may have a definition of a typical arterial system of a subject. This may be then deformed so that it fits within the arterial image.

In other examples the aforementioned methods of creating the arterial mask are combined or used separately and the results are combined to check and reduce the chance of air in creating the arterial mask.

In another example execution of the tagging inversion pulse portion by the magnetic resonance imaging system causes a tagging magnetization transfer effect in the resultant tagged magnitude image. The control inversion pulse portion causes a control magnetization transfer effect in the control magnitude image. The tagging magnetization transfer effect is extracted out of the arterial image by the control magnetization transfer effect. When performing arterial spin tagging it is normal practice to perform a tagged image and a control image. The construction of the pulse sequences in this way enables a better image of the artery system to be performed. The use of tagging and non-tagging to create tagged images and control images is described in section 17.1 of the Handbook of MRI Pulse Sequences.

In another aspect the invention provides for a computer program product comprising instructions for execution by a processor controlling the magnetic resonance imaging system for imaging a subject. Execution of the machine-executable instructions causes the processor to acquire tagged magnetic resonance data by controlling the magnetic resonance imaging system with tagging pulse sequence commands. The tagging pulse sequence commands comprise a tagging inversion pulse portion for spin labeling a tagging location within the subject. The tagging pulse sequence commands comprise a phase-contrast readout portion. The phase-contrast readout portion comprises phase-contrast encoding in at least one direction. Execution of the machine-executable instructions further cause the processor to acquire control magnetic resonance data by controlling the magnetic resonance imaging system with the control pulse sequence commands. The control pulse sequence commands comprise a control inversion pulse portion. The control pulse sequence commands comprise the phase-contrast readout portion. Execution of the machine-executable instructions further cause the processor to reconstruct a tagged magnitude image using the tagged magnetic resonance data.

Execution of the machine-executable instructions further cause the processor to reconstruct a control magnitude image using the control magnetic resonance data. Execution of the machine-executable instructions further cause the processor to construct an arterial image by subtracting the tagged magnitude image from the control magnitude image. Execution of the machine-executable instructions further cause the processor to reconstruct a phase using either the tagged magnetic resonance data and/or the control magnetic resonance data. There may be a phase image is reconstructed for each of the at least one direction.

In another aspect, the invention provides for a method of operating the magnetic resonance imaging system for imaging a subject. The method comprises acquiring tagged magnetic resonance data by controlling the magnetic resonance imaging system with tagging pulse sequence commands. The tagging pulse sequence commands comprise a tagging inversion pulse portion for spin labeling a tagging location within the subject. The tagging pulse sequence commands comprise a phase-contrast readout portion. The phase-contrast readout portion comprises phase-contrast encoding in at least one direction.

The method further comprises acquiring control magnetic resonance data by controlling the magnetic resonance imaging system with the control pulse sequence commands. The control pulse sequence commands comprise a control inversion pulse portion. The control pulse sequence commands comprise the phase-contrast readout portion. The method further comprises reconstructing a tagged magnitude image using the tagged magnetic resonance data. The method further comprises reconstructing a control magnitude image using the control magnetic resonance data.

The method further comprises constructing an arterial image by subtracting the tagged magnitude image from the control magnitude image.

The method further comprises reconstructing a phase image using either the tagged magnetic resonance data and/or the control magnetic resonance data. The phase image may be for each of the at least one direction.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as an apparatus, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer executable code embodied thereon.

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A ‘computer-readable storage medium’ as used herein encompasses any tangible storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. The computer-readable storage medium may also be referred to as a tangible computer readable medium. In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device. Examples of computer-readable storage media include, but are not limited to: a floppy disk, a magnetic hard disk drive, a solid state hard disk, flash memory, a USB thumb drive, Random Access Memory (RAM), Read Only Memory (ROM), an optical disk, a magneto-optical disk, and the register file of the processor. Examples of optical disks include Compact Disks (CD) and Digital Versatile Disks (DVD), for example CD-ROM, CD-RW, CD-R, DVD-ROM, DVD-RW, or DVD-R disks. The term computer readable-storage medium also refers to various types of recording media capable of being accessed by the computer device via a network or communication link. For example a data may be retrieved over a modem, over the internet, or over a local area network. Computer executable code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wire line, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

A computer readable signal medium may include a propagated data signal with computer executable code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device.

‘Computer memory’ or ‘memory’ is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. ‘Computer storage’ or ‘storage’ is a further example of a computer-readable storage medium. Computer storage is any non-volatile computer-readable storage medium. In some embodiments computer storage may also be computer memory or vice versa.

A ‘processor’ as used herein encompasses an electronic component which is able to execute a program or machine executable instruction or computer executable code. References to the computing device comprising “a processor” should be interpreted as possibly containing more than one processor or processing core. The processor may for instance be a multi-core processor. A processor may also refer to a collection of processors within a single computer system or distributed amongst multiple computer systems. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor or processors. The computer executable code may be executed by multiple processors that may be within the same computing device or which may even be distributed across multiple computing devices.

Computer executable code may comprise machine executable instructions or a program which causes a processor to perform an aspect of the present invention. Computer executable code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the C programming language or similar programming languages and compiled into machine executable instructions. In some instances the computer executable code may be in the form of a high level language or in a pre-compiled form and be used in conjunction with an interpreter which generates the machine executable instructions on the fly.

The computer executable code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

Aspects of the present invention are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It is understood that each block or a portion of the blocks of the flowchart, illustrations, and/or block diagrams, can be implemented by computer program instructions in form of computer executable code when applicable. It is further understood that, when not mutually exclusive, combinations of blocks in different flowcharts, illustrations, and/or block diagrams may be combined. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

A ‘user interface’ as used herein is an interface which allows a user or operator to interact with a computer or computer system. A ‘user interface’ may also be referred to as a ‘human interface device.’ A user interface may provide information or data to the operator and/or receive information or data from the operator. A user interface may enable input from an operator to be received by the computer and may provide output to the user from the computer. In other words, the user interface may allow an operator to control or manipulate a computer and the interface may allow the computer indicate the effects of the operator's control or manipulation. The display of data or information on a display or a graphical user interface is an example of providing information to an operator. The receiving of data through a keyboard, mouse, trackball, touchpad, pointing stick, graphics tablet, joystick, gamepad, webcam, headset, pedals, wired glove, remote control, and accelerometer are all examples of user interface components which enable the receiving of information or data from an operator.

A ‘hardware interface’ as used herein encompasses an interface which enables the processor of a computer system to interact with and/or control an external computing device and/or apparatus. A hardware interface may allow a processor to send control signals or instructions to an external computing device and/or apparatus. A hardware interface may also enable a processor to exchange data with an external computing device and/or apparatus. Examples of a hardware interface include, but are not limited to: a universal serial bus, IEEE 1394 port, parallel port, IEEE 1284 port, serial port, RS-232 port, IEEE-488 port, bluetooth connection, wireless local area network connection, TCP/IP connection, ethernet connection, control voltage interface, MIDI interface, analog input interface, and digital input interface.

A ‘display’ or ‘display device’ as used herein encompasses an output device or a user interface adapted for displaying images or data. A display may output visual, audio, and or tactile data. Examples of a display include, but are not limited to: a computer monitor, a television screen, a touch screen, tactile electronic display, Braille screen, Cathode ray tube (CRT), Storage tube, Bi-stable display, Electronic paper, Vector display, Flat panel display, Vacuum fluorescent display (VF), Light-emitting diode (LED) display, Electroluminescent display (ELD), Plasma display panel (PDP), Liquid crystal display (LCD), Organic light-emitting diode display (OLED), a projector, and Head-mounted display.

Magnetic Resonance (MR) data is defined herein as being the recorded measurements of radio frequency signals emitted by atomic spins using the antenna of a magnetic resonance apparatus during a magnetic resonance imaging scan. Magnetic resonance data is an example of medical image data. A Magnetic Resonance (MR) image is defined herein as being the reconstructed two or three dimensional visualization of anatomic data contained within the magnetic resonance imaging data. This visualization can be performed using a computer.

It is understood that one or more of the aforementioned embodiments of the invention and/or examples describe herein may be combined as long as the combined embodiments and/or examples are not mutually exclusive.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

FIG. 1 illustrates an example of a magnetic resonance imaging system;

FIG. 2 illustrates a further example of a magnetic resonance imaging system;

FIG. 3 shows a flow chart which illustrates a method of operating the magnetic resonance imaging system of FIG. 1 or FIG. 2;

FIG. 4 shows a timing chart which illustrates one TR of tagging pulse sequence commands;

FIG. 5 shows a timing chart which illustrates one TR of control pulse sequence commands;

FIG. 6 shows a flow chart which illustrates a method of producing a selective phase contrast angiogram;

FIG. 7 shows a flow chart which illustrates a method of visualizing a venous system;

FIG. 8 illustrates an example of a mapping of the right internal carotid artery;

FIG. 9 illustrates a mapping of three arteries in a brain;

FIG. 10 shows a flow chart which illustrates an example of a method;

FIG. 11 shows a flow chart which illustrates a further example of a method;

FIG. 12 illustrates a further example of a magnetic resonance imaging system; and

FIG. 13. Illustrates a method of operating the magnetic resonance imaging system of FIG. 12.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Like numbered elements in these figures are either equivalent elements or perform the same function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent.

FIG. 1 shows an example of a magnetic resonance imaging system 100. The magnetic resonance imaging system 100 comprises a magnet 104. The magnet 104 is a superconducting cylindrical type magnet 104 with a bore 106 through it. The use of different types of magnets is also possible. Inside the cryostat of the cylindrical magnet, there is a collection of superconducting coils. Within the bore 106 of the cylindrical magnet 104 there is an imaging zone 108 where the magnetic field is strong and uniform enough to perform magnetic resonance imaging.

Within the bore 106 of the magnet there is also a set of magnetic field gradient coils 110 which is used for acquisition of magnetic resonance data to spatially encode magnetic spins within the imaging zone 108 of the magnet 104. The magnetic field gradient coils 110 are connected to a magnetic field gradient coil power supply 112. The magnetic field gradient coils 110 are intended to be representative. Typically magnetic field gradient coils 110 contain three separate sets of coils for spatially encoding in three orthogonal spatial directions. A magnetic field gradient power supply supplies current to the magnetic field gradient coils. The current supplied to the magnetic field gradient coils 110 is controlled as a function of time and may be ramped or pulsed.

Adjacent to the imaging zone 108 is a radio-frequency coil 114 for manipulating the orientation of magnetic spins within the imaging zone 108 and for receiving radio transmissions from spins also within the imaging zone 108. The radio frequency antenna may contain multiple coil elements. The radio frequency antenna may also be referred to as a channel or antenna. The radio-frequency coil 114 is connected to a radio frequency transceiver 116. The radio-frequency coil 114 and radio frequency transceiver 116 may be replaced by separate transmit and receive coils and a separate transmitter and receiver. It is understood that the radio-frequency coil 114 and the radio frequency transceiver 116 are representative. The radio-frequency coil 114 is intended to also represent a dedicated transmit antenna and a dedicated receive antenna. Likewise the transceiver 116 may also represent a separate transmitter and receiver. The radio-frequency coil 114 may also have multiple receive/transmit elements and the radio frequency transceiver 116 may have multiple receive/transmit channels.

Within the bore 106 of the magnet 104 there is a subject support 120 which supports the subject in the imaging zone 108. A region of interest 109 can be seen within the imaging zone 108.

The transceiver 116, the magnetic field gradient coil power supply 112 are seen as being connected to a hardware interface 128 of computer system 126. The computer 126 further comprises a processor 130, a user interface 132, computer storage 134, and computer memory 136. The hardware interface 128 enables the processor 130 to send and receive commands and data in order to control the functioning of the magnetic resonance imaging system 100. The processor 130 is further connected to the user interface 132, the computer storage 134, and the computer memory 136.

The contents of the computer storage 134 and the computer memory 136 may be interchangeable. In some examples the contents of the computer storage 134 may be duplicated in the computer memory 136.

The computer storage 134 is shown as containing tagging pulse sequence commands 140. The computer storage 134 is further shown as containing control pulse sequence commands 142. The tagging pulse sequence commands 140 comprise a tagging inversion pulse portion for spin labeling the tagging location 122 within the subject 118. The tagging pulse sequence commands further comprise a phase-contrast readout portion. The phase-contrast readout portion comprises phase-contrast encoding in a first direction, a second direction, and a third direction. Normally these three directions would be aligned with the x, y and z-axis of the magnetic resonance magnet 104. This however is not necessary. The control pulse sequence commands comprise a control inversion pulse portion. The control pulse sequence commands 142 comprise the phase-contrast readout portion. The computer storage 134 is further shown as containing tagged magnetic resonance data 144 that has been acquired by controlling the magnetic resonance imaging system 100 using the tagging pulse sequence commands 140. The computer storage 134 is further shown as containing control magnetic resonance data 146 that has been acquired by controlling the magnetic resonance imaging system 100 with the control pulse sequence commands 142.

Pulse sequence commands as used herein encompass commands or a timing diagram which may be converted into commands which are used to control the functions of the magnetic resonance imaging system 100 as a function of time. Pulse sequence commands are the implementation of the magnetic resonance imaging protocol applied to a particular magnetic resonance imaging system 100.

Within the region of interest 109 there can be seen a tagging location 122. The tagging location is where the tagging inversion pulse portion labels a bolus of blood that passes through the arteries of the subject 118. In this case the region of interest 109 is shown as encompassing the head. The tagging location 122 in this case is a plane. The tagging is therefore non-selective and any blood passing through the plane 122 will be labeled. The positioning of the plane of the tagging location 122 near the neck of the subject 118 essentially means that all blood entering into the brain of the subject 118 will be effectively tagged. The example shown in FIG. 1 shows non-selective tagging.

The computer storage 134 further shows a tagged magnitude image 148 that has been reconstructed from the tagged magnetic resonance data. The computer storage 134 further shows a control magnitude image reconstructed from the control magnetic resonance data 146. When magnetic resonance data is acquired there may be different types of images which may be reconstructed. For example both magnitude and phase images may be reconstructed. In a normal clinical situation the magnitude image would indicate a density of hydrogen atoms at a particular location.

The computer storage 134 is further shown as containing an arterial image 152 that was constructed by subtracting the tagged magnitude image 150 from the control magnitude image 148. The computer storage 134 is further shown as containing an arterial mask 154. The arterial mask 154 is essentially an identification of the location of arteries within the arterial image 152. The arterial mask 154 can be constructed in different ways. A simple thresholding of the arterial image may be sufficient in some cases. In other examples more sophisticated methods such as a segmentation algorithm or fitting an anatomical atlas or model to the arterial image 154 may be used.

The computer storage 134 is further shown as containing a first phase image 156, a second phase image 154, and a third phase image 160. The first phase image 156, the second phase image 154, and the third phase image 160 may be reconstructed using the tagged magnetic resonance data 144 and/or the control magnetic resonance data 146 by using the phase encoding in the first, second, and third directions. The example illustrated in FIG. 1 is illustrative in that three encoding directions are used. In other examples there may be more or fewer encoding directions.

The computer storage 134 is further shown as containing a first intermediate arterial phase contrast image 156′ that was calculated by setting voxels of the first phase image that are outside of the arterial mask to a predetermined background value. The computer storage 134 is shown as further containing a second intermediate arterial phase contrast image 158′ which was calculated by setting voxels of the second phase image that are outside of the arterial mask to a predetermined background value. The computer storage 134 is further shown as containing a third intermediate arterial phase contrast image that was constructed or calculated by setting voxels of the third phase image that are outside of the arterial mask to a predetermined background value.

The computer storage is further shown as containing a selective phase contrast arterial image 162 that was calculated by adding the first intermediate arterial phase contrast image, the second intermediate arterial phase contrast image, the third intermediate arterial phase contrast image. If there are more or fewer phase encoding directions, there may be accordingly more or fewer selective phase contrast arterial images.

In other examples, the computer storage 134 may also contain a venous image 164. The venous image can created by applying the arterial mask 154 to the combined phase images 156, 158, 160 to exclude voxels that are identified as belonging to the arterial system of the subject 118. Remaining voxels above a predetermined threshold or value may then be interpreted as belonging to the vein system.

The computer storage 134 is further shown as containing a composite image 166 which is created by combining the venous image 164 with either the arterial image 152 or the selective phase contrast arterial image. In some cases the composite image may also be created by combining the venous image 164 with the arterial mask 154. The composite image 166 may in some cases use different colors or other emphasizing marks to differentiate between arteries and veins within the image.

FIG. 2 shows a further example of a magnetic resonance imaging system 200. The magnetic resonance imaging system 200 is similar to the system 100 shown in FIG. 1. However in this example there is more than one tagging location. The two tagging locations 122, 122′ may be positioned such that individual arteries are selectively tagged. This is an example of selective tagging. In this example the computer storage 134 is further shown as containing a selective phase contrast arterial image 162′ corresponding to an acquisition made when the tagging location is 122′. Arterial images corresponding to the tagging locations 122, 122′ can both be combined to make the arterial mask 154. In this way individual arteries can contribute to the arterial mask 154 and be subtracted from the venous image 164. The acquisition may be run an arbitrary number of times with different tagging locations 122, 122′ so that a composite image 166 can be made comprising contributions from an arbitrary number of arteries. For example different arteries could be color coded or marked in different ways in the image so that the blood contribution from individual arteries can be mapped within a region of the subject 118.

FIG. 3 shows a flowchart which illustrates a method of operating the magnetic resonance imaging system 100 of FIG. 1 or the magnetic resonance imaging system 200 of FIG. 2. First in step 300 the magnetic resonance imaging system is controlled with the tagging pulse sequence commands 140 to acquire the tagged magnetic resonance data 144. Next in step 302 the magnetic resonance imaging system 100 or 200 is controlled with the control pulse sequence commands 142 to acquire the control magnetic resonance data 146. Next in step 304 a tagged magnitude image 148 is reconstructed from the tagged magnetic resonance data 144. Next in step 306 the control magnitude image 150 is reconstructed from the control magnetic resonance data 146. Next in step 308 the arterial image 152 or 152′ is constructed by subtracting the tagged magnitude image 150 from the control magnitude image 148. Then in step 310 the arterial mask 154 is constructed using the arterial image 152 and identifying arteries in the arterial image 152. Next in step 312 the first phase image 156, the second phase image 158, and the third phase image 160 are reconstructed.

Optional steps may include step 314 where a first intermediate arterial phase contrast image 156′ by setting voxels of the first phase image that are outside of the arterial mask 154 to a predetermined background value. Step 314 may also include constructing a second intermediate arterial phase contrast image 158′ by setting voxels of the second phase image that are outside of the arterial mask 154 to a predetermined background value. Step 314 may further include constructing a third intermediate arterial phase contrast image 160′ by setting voxels of the third phase image that are outside of the arterial mask 154 to a predetermined background value. Step 316 may include calculating a selective phase contrast arterial image 162 by adding the first intermediate arterial phase contrast image, the second intermediate arterial phase contrast image, the third intermediate arterial phase contrast image.

Examples may provide for a method for artery-selective, non-contrast-enhanced Magnetic Resonance Angiography is presented. It may be based on selective Arterial Spin Labeled (ASL) Magnetic Resonance Imaging to visualize a single artery of interest in conjunction with phase encoded information to visualize blood flow velocity and direction acquired in a single acquisition. The selective images obtained by Arterial Spin Labeling may be analyzed and processed using information of the phase encoded acquisitions. Extracting the information of both, selective ASL and phase-contrast information with different directional encoding from one acquisition leads to the final images, which display velocity, flow direction and morphology of a single selected artery. Furthermore, the venous system can be visualized as well.

The selective visualization of intracranial or other arteries is an important differential diagnostic tool in radiological applications. Especially in neurovascular imaging, the information about a single selected artery (e.g. the carotid artery) can become crucial for an advanced diagnosis. Examples include intracranial cross-flow in patients with stenotic arteries, where the contralateral side supplies both hemispheres in part or in total. However, imaging all vessels at once would not suffice in finding this information. Another example are arterio-venous-malformations (AVM), as the identification of individual feeding arteries is crucial for the subsequent treatment planning Not only the identification and flow territories of selected arteries are of importance, but also hemodynamic properties, which include flow directions, identification of turbulent flow and display of blood flow velocity. Current gold standard methods, including X-Ray digital subtraction angiography (DSA), computed tomography angiography (CTA) and even magnetic resonance imaging (MRI) are currently limited in terms of delivering all the information in a timely manner and without exposing patients to any risk (e.g. ionizing radiation).

In MRI, the selective visualization of individual arteries can be achieved by applying pre-pulses using selective spatial saturation or selective arterial spin labeling (ASL) methods. ASL is based on the principle of subtracting two images; one is denoted as “control” (no inversion of blood spins) and the other as “label” (inversion of blood spins). Inversion of the magnetization of blood spins is achieved using dedicated RF pulses, which are applied during the inflow of blood. By subtracting these images, the static tissue cancels out. In selective methods, this is also the case for non-labeled arteries and only signal from the tagged artery is visible. ASL techniques make it possible to visualize vessel morphology and flow territories. Obtaining information about hemodynamics is also possible using time-resolved acquisitions; however, a quantification is limited. This is due to the fact that such acquisitions are limited in both spatial and temporal resolution in order to keep the measurement times to clinically acceptable durations.

Phase-contrast angiography (PCA) is a method which makes it possible to measure blood flow direction and velocity. The measurement and visualization of such parameters in only a single artery of interest is not possible. However, this is important, because in patients with cerebrovascular diseases the blood flow can be altered and an advanced diagnosis (e.g. intra-cranial crossflow) is impeded when all vessels are visualized at the same time. Furthermore, also venous flow is visualized in PCA measurements, potentially degrading image quality and reducing diagnostic confidence of the arterial vasculature.

Examples may enable the gathering of artery-selective information in conjunction with important hemodynamic parameters like blood flow direction and velocity in the cerebrovascular system in a single acquisition within a clinically acceptable scan time and by advanced analysis and processing of the acquired data.

At least two acquisitions are required by means of selective ASL and phase contrast angiography which prolong scan times significantly and make the measurements prone to subject motion. Additionally, arterial and venous signal can be separated in order to only visualize the arterial or the venous vasculature.

The gold standard method of obtaining selective angiograms is X-Ray digital subtraction angiography (DSA). This method relies on selectively placing an endovascular catheter through the iliac (or brachial) artery and injection of contrast agent. However, this method is invasive due to the need for placing an arterial access and the application of X-Rays and contrast agent. Other available image modalities such as computed tomography angiography (CTA) and contrast-enhanced MRA impede artery-selective imaging as these methods require the injection of exogenous contrast agents. After passing through the pulmonary arteries and then being ejected from the heart, the bolus traverses into the arteries (“arterial phase”) where image acquisition is subsequently performed as fast as possible to catch the first pass arterial inflow and visualize the hemodynamic properties. In fast CE-MRA methods, after the first pass of contrast agent has entered the brain, the later arriving arterial blood is visualized simultaneously with the venous outflow, which might impede the assessment of the arterial vasculature. While non-contrast enhanced methods pose an attractive alternative to contrast agent injection, they usually cannot be performed artery-selectively. The most commonly used method in Neuro-MRA is TOF Angiography, where the inflow of unsaturated arterial blood is used to form an image of the intracranial arteries. This method, however, only allows for static visualization of all arteries for morphological assessment of the cerebrovasculature, yet no information about a single artery can be obtained. Furthermore, no hemodynamic properties can be visualized or quantified.

Hemodynamic information can be obtained by using PCA. Here, information about flow is gathered by the application of a phase encoding gradient along the flow direction(s) of the arterial spins. Depending on the blood flow velocity, the individual spins experience a phase shift, which can be used to draw conclusions on the direction and velocity of the blood flow. Another method for the acquisition of intracranial arteries is based on Arterial Spin Labeling (ASL). The basic principle of ASL Angiography is the inversion of arterial water spins in the up streaming blood of a single artery or of all arteries at once. After acquisition of an image with inversion (label) and an image without inversion (control), subsequent subtraction results in angiograms with high SNR, as the background signal is ideally cancelled out. Despite making it possible to use ASL for selective and time-resolved imaging, detailed information about blood flow velocity or blood flow direction is not available.

In some examples, selective ASL and PCA information are obtained in a single acquisition, thereby allowing for artery selective velocity and flow imaging. The following contrasts can be generated from one ASL-PCA measurement:

Structural brain images (unsubtracted label/control acquisitions)

Directional encoded blood flow images (PCA phase acquisitions)

Static non-selective angiographic images (PCA magnitude image)

Static angiograms of selected arteries (subtracted label/control acquisitions)

-   -   A selective phase-contrast angiogram after applying the         generated image mask to the phase encoded images     -   An image of each artery with velocity information     -   A venous-only image after removing the arterial signal from the         PCA images     -   An example pulse sequence may comprise a 90° saturation module         (WET) is used in order to saturate the static tissue signal.         Then ASL labeling is performed for a defined amount of time         (e.g. 1000 ms). This time is chosen according to the blood flow         velocity so that all vessels are filled with previously inverted         blood at the time of image acquisition. Such information is         accessible from previous measurements in literature. During         readout (e.g. Gradient Echo) directional encoding is performed         using phase encoding gradients. Phase encoding can be performed         in one (e.g. right-left, feet-head, anterior-posterior)         direction or more. This experiment is performed twice, to         acquire complete label and control images as in conventional ASL         approaches. A schematic of the sequence structure is displayed         in FIGS. 4 and 5.

FIG. 4 is used to illustrate the tagging pulse sequence commands 140. FIG. 4 shows one TR or pulse repetition 400 of the tagging pulse sequence commands 140. The pulse repetition 400 comprises an optional pre-saturation portion 402, a tagging inversion pulse portion 404, and a phase-contrast readout 406. The pre-saturation portion 402 is not necessary but if it is used it may reduce the signal-to-noise and artefacts in the tagged magnitude image. The tagging inversion pulse portion 404 in this case is for selectively labeling a small volume and not for a plane. In FIG. 4, Selective labeling is realized using a selective ASL technique. In this approach, super-selective Arterial Spin Labeling is used. Readout is performed after sufficient filling of all arteries. The phase-encoding gradients are applied in one, two or in all three logical axes sequentially.

FIG. 5 shows an example of one pulse repetition 400 of a control pulse sequence command 142. The pulse repetition of FIG. 5 is similar to that of FIG. 140. It comprises the same preset optional pre-saturation portion 402, a control inversion pulse portion 500 and the same phase-contrast readout 406. The control inversion pulse portion 500 is similar to the tagging inversion pulse portion 404 except that the control inversion pulse portion 500 does not cause inversion within the tagging location. The control inversion pulse portion 500 may be chosen such that the magnetization transfer effect within the region of interest is essentially equivalent to the magnetization transfer effect caused by the tagging inversion pulse portion 404.

FIGS. 4 and 5 show the phase-contrast readout 406 as being part of the tagging pulse sequence commands 140 and the control pulse sequence commands 142. As an alternative, the tagging pulse sequence commands 140 and the control pulse sequence commands 142 do not contain the phase-contrast readout 406. In stead the phase-contrast readout is incorporated into different pulse sequence commands.

Post-processing of this data allows for artery-selective functional assessment of blood flow. From the ASL data, selective angiograms are generated using arithmetic image subtraction. From these angiograms, a binary mask is created, which is then applied to the three directional encoded images of the PCA dataset. Subsequent merging of these three images results in the final selective PCA images, presenting blood flow velocity in all phase encoding directions. A flow-chart of how image analysis and processing is performed is shown in FIGS. 6 and 7.

FIG. 6 shows a method of visualizing a single artery selectively and constructing an arterial image 152 for that artery and also for constructing a venous image 164 at the same time. After the tagged magnetic resonance data 144 and the control magnetic resonance data 146 have been acquired the tagged magnitude image 148 and the control magnitude image 150 are reconstructed. The labeled or tagged magnitude image 150 is then subtracted from the control magnitude image 148. This is used to calculate the arterial image 152. Then either by thresholding or using the more complicated segmentation technique the arterial mask 154 is calculated. The tagged magnetic resonance data or the control magnetic resonance data is then used to create three phase images in different directions. Shown are the first phase image 156, the second phase image 158 and the third phase image 160. The method may be formed in several different ways. For example the three phase images 156, 158, and 160 may first be combined and then the mask 154 may be applied. In the example shown in this Fig. the mask is applied independently to each of the three phase images first, 156, 158, 160. Image 156′ is the masked version 156′ of the first phase image 156. Image 158′ is a masked version of the second phase image 158 and image 160′ is a masked version of image 160. Only the voxels within the mask region 154 are taken. These three mask images which represent flow of blood in the three different directions may then be combined into a selected phase-contrast arterial image 600. For example different colors or other labels within the image 600 could be used to represent blood flow within the different directions.

FIG. 6 shows a flowchart displaying how image data analysis and processing is performed for the visualization of the right internal carotid artery. From the magnitude images, the static selective ASL angiography can be obtained. This image is then post-processed (binary mask creation) and applied to the non-selective phase-contrast images. A combination of all phase encoded directions results in the final images.

FIG. 7 illustrates how the venous system can be analyzed. In this example three different arteries are labeled selectively. They were labeled using the tagging volumes 122, 122′ and 122″. From each of the magnetic resonance data a control 150 and magnitude image 148 are reconstructed. From each pair of tagged magnitude image 148 and control magnitude image 150 an arterial image 152, 152′ and 152″ are calculated. Then from each of these arterial images a mask 154, 154′, 154″ is created. These may then be all merged into a composite arterial mask 700. As with FIG. 6, the three phase images 156, 158, 160 in the three different directions are reconstructed. Then the composite arterial mask 700 is applied to each of the phase images 156, 158 and 160. In this case voxels within the arterial mask in the corresponding phase images 156, 158, and 160 are set to a predetermined value such as zero. After the mask has been applied to each of the images 156, 158, 160, the three images are then merged or combined to form the venous image 164. The venous image 164 then shows a mapping of the flow of blood in different directions. For example a color coding system can be used to indicate the direction of blood flow within a three-dimensional image.

FIG. 8 displays how image analysis and processing is performed to only visualize the venous system. From the magnitude images of all selectively labelled arteries, the static selective ASL angiography images can be obtained. These images are then post-processed (binary mask creation) and put into one frame in order to generate a mask of the complete arterial vasculature. Subsequently, this mask is applied to the non-selective phase-contrast images in order to discriminate arterial and venous vessels. Finally, the individual phase encoded images (RL, AP, FH) presenting only venous information can be combined into one image.

Obtainable image contrast:

Primarily, magnitude images for the evaluation of anatomical structures are readily available. Additionally, directional phase encoded images, usually performed in all three logical axis of the scanner coordinate system are available. These images are also used to visualize non-selective static angiography images.

Post-processing of the ASL data is performed as subtraction of the control and label images, so that static background signal and the non-labeled arteries are cancelled out, leaving only signal of the labeled artery. This is performed for each labeled artery. From these angiograms, a binary mask is created, which is applied to the three directional encoded images from the PCA data. To generate this mask, several techniques can be used. These include for example thresholding of the source images, the use of segmentation algorithms or discrimination by the measured velocity (i.e. fast velocities correspond to arteries, while slow velocities are identified as veins). Subsequent merging of these three masked images results in the final images, because all unwanted (contralateral and venous) vessel information is discarded after application of the image mask. The final images present velocity information and flow direction for individually selected arteries only. A flow-chart of how data analysis and processing are performed is shown in FIG. 6.

To acquire images of the venous system, the data analysis and processing is performed differently. Hence, the information about all cerebral arteries is needed as the localization of arteries can be used to indirectly obtain the location of the veins. For that purpose, all artery-selective images have to be combined into a single mask to display the whole arterial tree. Applying this information on the phase images, it is possible to visualize only venous flow. This flowchart is displayed in FIG. 7.

Image representation can be performed in several ways. For example, the directional information is presented in a single artery or the velocity information for each single artery is shown on a single image with individual colorbars representing velocity of each artery. These examples, without colorbars, are shown in FIGS. 8 and 9.

FIG. 8 shows an example of a selective phase-contrast arterial image 600. FIG. 8 is a projection of a three-dimensional image onto the paper or plane. FIG. 8 shows the directional mapping of the right internal carotid artery. The anterior posterior direction is displayed in red in the original three-dimensional image. The right left direction may be displayed in blue and the feet head flow direction may be displayed in green. However, color is not shown in the projection of FIG. 8.

FIG. 9 shows a further example of a selective phase-contrast arterial image 900. In this example velocity mapping of three major brain feeding arteries in a healthy volunteer is displayed. The image shown in FIG. 9 is a projection of the three-dimensional image onto a plane. In the original three-dimensional image the right internal carotid artery is displayed in red, the left internal carotid artery is displayed in green and the posterior circulation is displayed in blue. The intensity of these different colors was scaled to show the velocity of blood through each of the arteries. This however is not displayed on FIG. 9. Dashed lines are used to indicate which portion of FIG. 9 belongs to the right internal carotid artery 902, the left internal carotid artery 904, and the posterior circulation 906. There is a region that overlaps both region 902 and 904 where some of the blood flow is due to the right internal carotid artery 902 and part of the blood flow is due to the left internal carotid artery 904. FIG. 9 illustrates a velocity mapping of the three major brain feeding arteries in a healthy volunteer. The right internal carotid artery is displayed in red, the left in green and the posterior circulation in blue. The intensity is scaled according to the blood flow velocities.

Applications include cerebrovascular diseases with complex and diffuse flow patterns, for which not only the selective information about an artery is important, but also underlying hemodynamic properties. These are especially arterio-venous malformations (AVM), but also fistulas, and tumor feeding arteries. Other applications include stenotic arteries, potentially leading to stroke.

Venous diseases can also be visualized separately by this method, as the information can be extracted from the data using the pre-processed arterial information. Here, applications could include venous stenosis (e.g. in pseudo-tumor cerebri patients) or thrombosis in the sinus veins.

Examples are not necessarily limited to the cerebral vasculature, but might also be used to visualize other arteries. These include selective visualization of the renal arteries, the coronary arteries, as well as the peripheral lower leg arteries.

FIG. 10 shows an example of a method. First in step 10 step 1000 is performed. In step 1000 a tagged ASL magnitude image of a region of interest of the subject is received. Next in step 1002 a control ASL magnitude image of the region of interest of the subject is received. Next in step 1004 an arterial image is constructed by subtracting the tagged ASL magnitude image from the control ASL magnitude image. Then in step 1006 an arterial mask is constructed using the arterial image by identifying arteries in the arterial image. Then in step 1008 a phase-contrast magnetic resonance image of at least a portion of the region of interest of the subject is received. Finally in step 1010 a selected phase-contrast arterial image is constructed at least partially by setting voxels of the phase-contrast image within the at least a portion of the region of interest that are outside of the arterial mask to a predetermined background value.

FIG. 11 shows a further example of a method. The method shown in FIG. 11 is similar to that in FIG. 10. However, instead of performing step 1010 after step 1008 step 1100 is performed instead. After step 1008 is performed step 1100 is performed where a venous image is constructed at least partially by setting the voxels of the phase-contrast image within the at least a portion of the region of interest that are within the arterial mask to a predetermined background value.

The methods illustrated in FIGS. 10 and 11 may be performed in an automated fashion by having them performed by a processor. The methods shown in FIGS. 10 and 11 may be combined into a single method where either step 1010 or step 1100 is performed first.

FIG. 12 shows a further example of a magnetic resonance imaging system 1200. The magnetic resonance imaging system 1200 of FIG. 12 is similar to the magnetic resonance imaging system 100 of FIG. 1. The main difference between the magnetic resonance imaging system 100 of FIG. 1 and the magnetic resonance imaging system 1200 of FIG. 12 is that the pulse sequences within the memory 134 or storage are different. The computer storage 134 is shown as containing tagging pulse sequence commands 140′, control pulse sequence commands 142′, and phase-contrast pulse sequence commands 1202. In the example in FIG. 12 the phase-contrast readout portion is within a separate pulse sequence 1202 and is not incorporated into the pulse sequences 140′ and 142′. The computer storage 134 is additionally shown as containing tagged magnetic resonance data 144′ that was acquired using the tagging pulse sequence commands 140′. The computer storage 134 is further shown as containing control magnetic resonance data 146′ that was acquired using the control pulse sequence commands 142′. The computer storage is further shown as containing phase-contrast magnetic resonance data 1204 that was acquired using the phase-contrast pulse sequence commands 1202.

The computer storage 134 is again shown as containing a tagged magnitude image 148 that was reconstructed from the tagged magnetic resonance data 144′. The computer storage 134 is further shown as containing a control magnitude image that was reconstructed from the control magnetic resonance data 146′. The computer storage 134 is further shown as containing a first phase image 156. The first phase image 156 was reconstructed from the phase-contrast magnetic resonance data 1204. In the example shown in FIG. 12 only the first phase image 156 is shown. If there are multiple phase encoding directions there may be multiple phase images such as are shown in FIG. 1.

The computer storage 134 is further shown as containing an arterial image 152 that was constructed by subtracting the tagged magnitude image 150 from the control magnitude image 148. The computer storage 134 is shown as containing an arterial mask 154 that was reconstructed from the arterial image 152. The computer storage 134 is shown as containing a selected phase-contrast arterial image 162, a venous image 164, and a composite image 166 that was constructed from both the venous image 164 and the selected phase-contrast arterial image 162. The arterial mask 154, the selected phase-contrast arterial image 162, the venous image 164, and the composite image 166 were reconstructed in the same way as the corresponding images in FIGS. 1 and 2. If there is a single phase image 156 as is shown in FIG. 12 then the arterial 162 and venous image 164 are constructed by applying the arterial mask 154 to the single phase image 156. If there are multiple phase images then the arterial mask 154 is applied to each of the phase images and then they are then summed or further processed to create the selected phase-contrast arterial image 162 and/or the venous image 164. The image example in FIG. 12 shows the single tagging location 122. However, the example shown in FIG. 12 could be modified in an equivalent way in which FIG. 2 is modified. The example shown in FIG. 12 may also incorporate multiple tagging locations 122, 122′.

FIG. 13 shows a method in a flow chart that illustrates how to operate the magnetic resonance imaging system 1200 of FIG. 12. First in step 1300 the tagged magnetic resonance data 144′ is acquired by controlling the magnetic resonance imaging system 1100 with the tagging pulse sequence commands 140′. Next in step 1302 the control magnetic resonance data 146′ is acquired by controlling the magnetic resonance imaging system 1100 with the control pulse sequence commands 142′. Next in step 1304 the phase-contrast magnetic resonance data 1204 is acquired by controlling the magnetic resonance imaging system 1200 with the phase-contrast pulse sequence commands 1202.

Next in step 1000 the tagged ASL magnitude image 148 is reconstructed using the tagged magnetic resonance data 144′. Next in step 1002 the control ASL magnitude image 150 is reconstructed using the control magnetic resonance data 146′. Next in step 1004 the arterial image 152 is constructed by subtracting the tagged ASL magnitude image 150 from the control ASL magnitude image 148. Next in step 1006 the arterial mask 154 is constructed using the arterial image 152 by identifying arteries in the arterial image. Next step 1008 phase-contrast image 156 is reconstructed using the phase-contrast magnetic resonance data 1204. Then finally in step 1010 the selected phase-contrast arterial image 162 is constructed at least partially by setting voxels of the phase-contrast image 156 that are within the portion of the region of interest of the tagged ASL magnitude image or the control ASL magnitude image and that are outside of the arterial mask 154 to a predetermined background value. As an alternative the selected phase-contrast arterial image may have voxels that are inside of the arterial mask to produce the venous image.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE NUMERALS

-   100 magnetic resonance system -   104 magnet -   106 bore of magnet -   108 imaging zone -   109 region of interest -   110 magnetic field gradient coils -   112 magnetic field gradient coil power supply -   114 radio-frequency coil -   116 transceiver -   118 subject -   120 subject support -   122 tagging location -   122′ tagging location -   126 computer system -   128 hardware interface -   130 processor -   132 user interface -   134 computer storage -   136 computer memory -   140 tagging pulse sequence commands -   140′ tagging pulse sequence commands -   142 control pulse sequence commands -   142′ control pulse sequence commands -   144 tagged magnetic resonance data -   144′ tagged magnetic resonance data -   146 control magnetic resonance data -   146′ control magnetic resonance data -   148 tagged magnitude image -   150 control magnitude image -   152 arterial image -   154 arterial mask -   156 first phase image -   156′ first intermediate arterial phase contrast image -   158 second phase image -   158′ second intermediate arterial phase contrast image -   160 third phase image -   160′ third intermediate arterial phase contrast image -   162 selective phase contrast arterial image -   162′ selective phase contrast arterial image -   164 venous image -   166 composite image -   170 machine executable instructions -   200 magnetic resonance imaging system -   300 acquire tagged magnetic resonance data by controlling the     magnetic resonance imaging system with the tagging pulse sequence     commands -   302 acquire control magnetic resonance data by controlling the     magnetic resonance imaging system with the control pulse sequence     commands -   304 reconstruct a tagged magnitude image using the tagged magnetic     resonance data -   306 reconstruct a control magnitude image using the control magnetic     resonance data -   308 construct an arterial image by subtracting the tagged magnitude     image from the control magnitude image -   310 construct an arterial mask using the arterial image by     identifying arteries in the arterial image -   312 reconstruct a first phase image, a second phase image, and a     third phase image using either the tagged magnetic resonance data     and/or the control magnetic resonance data, wherein the first phase     image is reconstructed using phase encoding in the first direction,     wherein the second phase image is reconstructed using phase encoding     in the second direction; -   314 construct a first intermediate arterial phase contrast image, a     second intermediate arterial phase contrast image, and a third     intermediate arterial phase contrast image by setting voxels of the     first phase image, the second phase image, and the that are outside     of the arterial mask to a predetermined background value -   316 calculate a selective phase contrast arterial image by adding     the first intermediate arterial phase contrast image, the second     intermediate arterial phase contrast image, the third intermediate     arterial phase contrast image -   400 pulse repetition -   402 presaturation portion -   404 tagging inversion pulse portion -   406 phase contrast readout -   500 control inversion pulse portion -   600 selective phase contrast angiogram -   700 composite arterial mask -   900 selective phase contrast angiogram -   902 blood flow in right internal carotid artery -   904 blood flow in left internal carotid artery -   906 blood flow in posterior circulation -   1000 receive a tagged ASL magnitude image of a region of interest of     a subject -   1002 receive a control ASL magnitude image of the region of interest     of the subject -   1004 construct an arterial image by subtracting the tagged ASL     magnitude image from the control ASL magnitude image -   1006 construct an arterial mask using the arterial image by     identifying arteries in the arterial image -   1008 receive a phase contrast magnetic resonance image of at least a     portion of the region of interest of the subject -   1010 construct a selective phase contrast arterial image at least     partially by setting voxels of the phase contrast image within the     at least a portion of the region of interest that are outside of the     arterial mask to a predetermined background value -   1100 construct a venous image at least partially by setting voxels     of the phase contrast image within the at least a portion of the     region of interest that are within the arterial mask to a     predetermined background value -   1200 magnetic resonance imaging system -   1202 phase-contrast pulse sequence commands -   1204 phase contrast magnetic resonance data -   1300 acquire tagged magnetic resonance data by controlling the     magnetic resonance imaging system with the tagging pulse sequence     commands -   1302 acquire control magnetic resonance data by controlling the     magnetic resonance imaging system with the control pulse sequence     commands -   1304 acquire phase-contrast magnetic resonance data by controlling     the magnetic resonance imaging system with the phase-contrast pulse     sequence commands 

The invention claimed is:
 1. A medical imaging system, wherein the medical imaging system comprises: a memory for storing machine executable instructions, and a processor for controlling the medical imaging system, wherein execution of the machine executable instructions causes the processor to: receive a tagged arterial spin labeled (ASL) magnitude image of a region of interest of a subject; receive a control arterial spin labeled (ASL) magnitude image of the region of interest of the subject; construct an arterial image by subtracting the control ASL magnitude image and the tagged ASL magnitude image; construct an arterial mask using the arterial image by identifying arteries in the arterial image; receive a phase contrast magnetic resonance image of at least a portion of the region of interest of the subject; and construct a venous image at least partially by setting voxels of the phase contrast image within the at least a portion of the region of interest that are within the arterial mask to a predetermined background value.
 2. The medical imaging system of claim 1, wherein execution of the machine executable instructions further cause the processor to construct a selective phase contrast arterial image at least partially by setting voxels of the phase contrast image within the at least a portion of the region of interest that are outside of the arterial mask to the predetermined background value.
 3. The medical imaging system of claim 2, wherein execution of the machine executable instructions cause the processor to calculate a composite image by combining the arterial image and the venous image into a composite image.
 4. The medical imaging system of claim 1, wherein the medical imaging system is a magnetic resonance imaging system, wherein the memory further contains phase-contrast pulse sequence commands, wherein the memory further contains tagging pulse sequence commands, wherein the memory further contains control pulse sequence commands, wherein the tagging pulse sequence commands comprise a tagging inversion pulse portion for spin labeling a tagging location within the subject, wherein the control pulse sequence commands comprise a control inversion pulse portion, wherein the phase-contrast pulse sequence commands comprise a phase-contrast readout portion, wherein the phase-contrast readout portion comprises phase-contrast encoding in at least one direction, wherein execution of the machine executable instructions further cause the processor to: acquire tagged magnetic resonance data by controlling the magnetic resonance imaging system with the tagging pulse sequence commands; acquire control magnetic resonance data by controlling the magnetic resonance imaging system with the control pulse sequence commands; reconstruct the tagged arterial spin labeled (ASL) magnitude image using the tagged magnetic resonance data, wherein the tagged ASL magnitude image is received by reconstructing the tagged ASL magnitude image; reconstruct the control arterial spin labeled (ASL) magnitude image using the control magnetic resonance data, wherein the control ASL magnitude image is received by reconstructing the control ASL magnitude image; acquire phase-contrast magnetic resonance data by controlling the magnetic resonance imaging system with the phase-contrast pulse sequence commands; and reconstruct the phase contrast image using the phase contrast magnetic resonance data at least partially using phase encoding in the at least one direction, wherein the phase contrast image is received by reconstructing the phase contrast image.
 5. The medical imaging system of claim 4 wherein the tagging location is spatially selective for tagging a spatial region containing a single artery.
 6. The medical imaging system of claim 5, wherein execution of the machine executable instructions cause the processor to repeat acquiring the tagged magnetic resonance data and the control magnetic resonance data for a predetermined number of tagging volumes, and wherein execution of the machine executable instructions causes the processor to calculate the arterial image for each of the predetermined number of tagging volumes.
 7. The medical imaging system of claim 4, wherein the tagging pulse sequence is not spatially selective.
 8. The medical imaging system of claim 4, wherein execution of the tagging inversion pulse portion by the magnetic resonance imaging system causes a tagging magnetization transfer effect in the tagged magnitude image, and wherein execution of the control inversion pulse portion by the magnetic resonance imaging system causes a control magnetization transfer effect in the control magnitude image, and wherein the tagging magnetization transfer effect is subtracted out of the arterial image by the control magnetization transfer effect.
 9. The medical imaging system of claim 4 wherein execution of the machine executable instructions further causes the processor to receive a registration between the phase contrast image and the control ASL magnitude image and/or the tagged ASL magnitude image, wherein the identification of voxels of the phase contrast image within the at least a portion of the region of interest that are outside of the arterial mask to the predetermined background value is at least partially performed using the registration.
 10. The medical imaging system of claim 9, wherein execution of the machine executable instructions further cause the processor to receive the registration between the phase contrast image and the control ASL magnitude image and/or the tagged ASL magnitude image by calculating a mapping between voxels of the phase contrast image and the control ASL magnitude image and/or the tagged ASL magnitude image.
 11. The medical imaging system of claim 1, wherein the medical imaging system is a magnetic resonance imaging system, wherein the memory further contains tagging pulse sequence commands and control pulse sequence commands, wherein the tagging pulse sequence commands comprise a tagging inversion pulse portion for spin labeling a tagging location within the subject, wherein the tagging pulse sequence commands comprise a phase-contrast readout portion, wherein the phase-contrast readout portion comprises phase-contrast encoding in at least one direction, wherein the control pulse sequence commands comprise a control inversion pulse portion, wherein the control pulse sequence commands comprises the phase-contrast readout portion, wherein execution of the machine executable instructions further cause the processor to: acquire tagged magnetic resonance data by controlling the magnetic resonance imaging system with the tagging pulse sequence commands; acquire control arterial spin labeled (ASL) magnetic resonance data by controlling the magnetic resonance imaging system with the control pulse sequence commands; reconstruct the tagged arterial spin labeled (ASL) magnitude image using the tagged magnetic resonance data, wherein the tagged ASL magnitude image is received by reconstructing the tagged ASL magnitude image; reconstruct the control magnitude image using the control magnetic resonance data, wherein the control ASL magnitude image is received by reconstructing the control ASL magnitude image; reconstruct the phase image using either the tagged magnetic resonance data and/or the control magnetic resonance data at least partially using phase encoding in the at least one direction, wherein the phase contrast image is received by reconstructing the phase contrast image.
 12. The magnetic resonance imaging system of claim 1, wherein the arterial mask is created using the arterial image according to any one of the following: by thresholding the arterial image using a predetermined threshold value, wherein portions of the arterial image above the predetermined threshold value are identified as being arterial regions; by segmenting the arterial image; by applying an anatomical atlas to the arterial image; by fitting a deformable shape model to the arterial image; and by combinations thereof.
 13. A computer program product comprising machine executable instructions for execution by a processor, wherein execution of the machine executable instructions causes the processor to: receive a tagged arterial spin labeled (ASL) magnitude image of a region of interest of a subject; receive a control ASL magnitude image of the region of interest of the subject; construct an arterial image by subtracting the control ASL magnitude image and the tagged ASL magnitude image; construct an arterial mask using the arterial image by identifying arteries in the arterial image; receive a phase contrast magnetic resonance image of at least a portion of the region of interest of the subject; and construct a venous image at least partially by setting voxels of the phase contrast image within the at least a portion of the region of interest that are within the arterial mask to a predetermined background value.
 14. The computer program product of claim 13, wherein execution of the machine executable instructions further causes the processor to construct a selective phase contrast arterial image at least partially by setting voxels of the phase contrast image within the at least a portion of the region of interest that are outside of the arterial mask to the predetermined background value.
 15. A method of medical imaging, wherein the method comprises: receiving a tagged arterial spin labeled (ASL) magnitude image of a region of interest of a subject; receiving a control ASL magnitude image of the region of interest of the subject; constructing an arterial image by subtracting the control ASL magnitude image and the tagged ASL magnitude image; constructing an arterial mask using the arterial image by identifying arteries in the arterial image; receiving a phase contrast magnetic resonance image of at least a portion of the region of interest of the subject; and constructing a venous image at least partially by setting voxels of the phase contrast image within the at least a portion of the region of interest that are within the arterial mask to a predetermined background value. 